A single carrier transmission process has widely been accepted as an uplink wireless access process beyond 3G (third generation). The single carrier transmission process is described in Non-patent Document 1 (Physical Layer Aspects for Evolved UTRA” (3GPP TR25.814v0.5.0 (2005-11), Chapter 9.1)), for example. FIG. 2 shows the arrangement of a transmitter according to the single carrier transmission process proposed in Non-patent Document 1, and FIG. 3 shows the arrangement of a general receiver compatible with the transmitter. A summary of operation of the transmitter and the receiver of the background art will be described below with reference to FIGS. 2 and 3.
First, data processing circuit 110 and pilot processing circuit 120 of the transmitter will be described below with reference to FIG. 2.
In data processing circuit 110, DFT (Discrete Fourier Transformation) (converting a time-domain signal into a frequency-domain signal) circuit 111 performs an Ntx_d point DFT computation on a data signal, thereby converting the data signal into a frequency-domain signal. The data signal is delimited to a data block size (=the number of subcarriers before the data signal passes through a transmission filter=Ntx_d) in which coded data symbols are transmitted as one block, and comprises Ntx_d symbols.
Roll-off filter circuit 112 performs a roll-off filtering process on the frequency-domain data signal in the frequency domain. Subcarrier mapping circuit 113 maps the data signal that has passed through roll-off filter circuit 112 onto subcarriers (specified for respective users). “0” is inserted into subcarriers that are not specified for respective users. Subcarrier mapping circuit 113 generates a data signal having a total of Ndft_d symbols.
IDFT (Inverse Discrete Fourier Transformation) (converting a frequency-domain signal into a time-domain signal) circuit 114 performs an Ndft_d point IDFT computation on the subcarrier-mapped data signal (signal comprising Ndft_d subcarriers), thereby converting the data signal back into a time-domain signal described
Cyclic prefix adding circuit 115 adds a cyclic prefix to each of the IDFTed blocks. The cyclic prefix is added to allow the receiver to effectively carry out a frequency domain equalizing process. The addition of a cyclic prefix refers to a process of copying a rear portion of a block to a front portion of the block, as shown in FIG. 1.
Pilot processing circuit 120 will be described below.
In pilot processing circuit 120, DFT circuit 121 performs an Ntx_p point DFT computation on pilot signal symbols (the number of symbols=the number of subcarriers before a pilot signal passes through the transmission filter=the length of a pilot sequence=Ntx_p) transmitted as one block, thereby converting the pilot signal into a frequency-domain signal.
The processing operation of pilot processing circuit 120 subsequent to DFT circuit 121 is the same as the above processing operation on the data signal. Specifically, roll-off filter circuit 122 performs a roll-off filtering process on the frequency-domain pilot signal. Subcarrier mapping circuit 123 performs a subcarrier mapping process on the pilot signal after it is processed by roll-off filter circuit 122. IDFT circuit 124 performs an Ndft_p point IDFT computation on the subcarrier-mapped pilot signal (signal comprising Ndft_p subcarriers), thereby converting the pilot signal back into a time-domain signal. Cyclic prefix adding circuit 125 adds a cyclic prefix to each block of the IDFTed signal.
Time-division multiplexing circuit 130 time-division-multiplexes the data signal and the pilot signal to which the cyclic prefixes are added, and transmits the time-division-multiplexed signal to the receiver. According to Non-patent Document 1, Ntx_p Ntx_d/2.
Data processing circuit 150 and pilot processing circuit 160 of the receiver will be described below with reference to FIG. 3.
In the receiver, cyclic prefix removing/data pilot separating circuit 140 removes the cyclic prefixes from the received signal and demultiplexes the time-division-multiplexed signal to separate the received data part and the received pilot part from each other. Cyclic prefix removing/data pilot separating circuit 140 outputs the received data part to data processing circuit 150 and also outputs the received pilot part to pilot processing circuit 160.
In pilot processing circuit 160, DFT circuit 161 performs an Ndft_p point DFT computation on the received pilot part separated by cyclic prefix removing/data pilot separating circuit 140, thereby converting it into a received pilot signal in the frequency domain. Subcarrier demapping circuit 162 demaps the subcarriers of the received pilot signal in the frequency domain, thereby extracting only the subcarriers transmitted by the user. Roll-off filter circuit 163 performs a roll-off filtering process on the subcarrier-demapped received pilot signal. Propagation channel estimating circuit 164 estimates a propagation channel for each subcarrier, using the received pilot signal that has passed through roll-off filter circuit 163.
Frequency-domain equalizing circuit 165 performs a frequency-domain equalizing process (for equalizing subcarriers in the frequency domain by multiplying the subcarriers by respective weighting coefficients) on the received pilot signal, using the propagation channels estimated by propagation channel estimating circuit 164. IDFT circuit 166 performs an Ndft_p point IDFT computation on the received pilot signal that has passed through frequency-domain equalizing circuit 165, thereby converting the received pilot signal back into a received pilot signal in the time domain. Amplitude estimating circuit 167 estimates an amplitude of the time-domain received pilot signal. The estimated amplitude value is used to demodulate the received data signal.
The estimated amplitude value in the time domain is calculated according to the following equation (1):
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                      Estimated          ⁢                                          ⁢          amplitude          ⁢                                          ⁢          value          ⁢                                          ⁢          in          ⁢                                          ⁢          time          ⁢                                          ⁢          domain                =                              1                          N              tx_p                                ⁢                                    ∑                              n                =                0                                            N                                  tx_p                  -                  1                                                      ⁢                                                  ⁢                                          a                n                *                            ⁢                              b                n                                                                        (        1        )            where an (n=0, 1, . . . , Ntx_p−1) represents a transmitted pilot sequence (time-domain), * a complex conjugate, and bn (n=0, 1, . . . , Ntx_p−1) a value of the equalized received pilot sequence in the time domain.
In data processing circuit 150, as with the above processing operation on the pilot signal, DFT circuit 151 performs an Ndft_d point DFT computation on the received data part separated by cyclic prefix removing/data pilot separating circuit 140, thereby converting it into a received data signal in the frequency domain. Subcarrier demapping circuit 152 demaps the subcarriers of the received data signal in the frequency domain, retrieving only the subcarriers transmitted by the user. Roll-off filter circuit 153 performs a roll-off filtering process on the subcarrier-demapped received data signal. Frequency-domain equalizing circuit 154 performs a frequency-domain equalizing process on each of the subcarriers of the received data signal that has passed through roll-off filter circuit 153. IDFT circuit 155 performs an Ntx_d point IDFT computation on the received data signal that has passed through frequency-domain equalizing circuit 154, thereby converting the received data signal back into a received data signal in the time domain. Demodulating circuit 156 demodulates the received data signal in the time domain, which has been equalized in the frequency domain, using the estimated amplitude value obtained from pilot processing circuit 160.
For the above wireless transmission, multilevel modulation (QAM: Quadrature Amplitude Modulation) is performed for increasing the transmission efficiency. In a wireless propagation channel, a transmitted signal that is multilevel-modulated suffers amplitude fluctuation and phase rotation peculiar to the wireless propagation channel, called fading.
In order for the receiver to demodulate and decode the received signal properly, it is necessary to estimate these variations (estimated propagation channel values and estimated amplitude values) properly (see, for example, Japanese Patent Laid-Open No. 2004-260774). The frequency domain equalizing process and the demodulating process described above are of general nature.
Generally, the complexity of DFT and IDFT computations depends on the number of points in the DFT and IDFT computations. Specifically, if the number of points is represented by a power of 2 (2, 4, 8, . . . , 512, 1024, . . . ), then the complexity of the computations is minimum. Conversely, if the number of points is represented by a prime number or a number including a large prime number, then the complexity of the computations is large. In view of the amount of DFT and IDFT computations, therefore, the numbers of points Ndft_d (data) and Ndft_p (pilot) in the IDFT computations performed by the transmitter and the DFT computations performed by the receiver should desirably be set to powers of 2. According to Non-patent Document 1, for example, if the system bandwidth is of 5 MHz, then Ndft_d is set to Ndft_d 512 (=the ninth power of 2) and Ndft_p is set to Ndft_p=256 (=the eighth power of 2). If the relationship Ndft_p=Ndft_d/2 is satisfied, then it means that the subcarrier interval of the pilot signal is twice the subcarrier interval of the data signal.
According to Non-patent Document 1, since the subcarrier interval of the pilot signal is twice the subcarrier interval of the data signal, the data block size Ntx_d and the pilot block size Ntx_p are set to satisfy Ntx_p=Ntx_d/2.
For pilot signals, attention has been attracted to a Zadoff-Chu sequence which is one of “CAZAC (Constant Amplitude Zero AutoCorrelation) sequences”.
A Zadoff-Chu sequence ck is expressed by the following equation (2):
                    [                  Equation          ⁢                                          ⁢          2                ]                                                                      c                      k            ⁢                                                                =                  exp          ⁡                      [                                          -                                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    πk                                    Ntx_p                                            ⁢                              (                                                      n                    ⁢                                                                  n                        +                        1                                            2                                                        +                  qn                                )                                      ]                                              (        2        )            
In the equation (2), n represents 0, 1, . . . , Ntx_p−1, and q an arbitrary integer. The Zadoff-Chu sequence is described in detail in Non-patent Document 2 (K. Fazel and S. Keiser, “Multi-Carrier and Spread Spectrum Systems” (John Wiley and Sons, 2003)).
“CAZAC sequence” is a signal sequence which has a constant amplitude in the time domain and the frequency domain and whose periodic autocorrelation value is 0 with respect to time shifts other than 0. According to “CAZAC sequence”, since its amplitude is constant in the time domain, PAPR (Peak to Average Power Ratio) can be reduced. Since the amplitude is constant also in the frequency domain, “CAZAC sequence” is a signal sequence which is suitable for the estimation of a propagation channel in the frequency domain. Reduced PAPR means a reduction in the power consumption, which is preferable particularly for mobile communications.
Furthermore, inasmuch as “CAZAC sequence” has perfect autocorrelation, it is advantageous in that allows the timing of a received signal to be detected with ease, and is drawing attention as a pilot sequence suitable for use in a single carrier transmission process as an uplink wireless access process beyond 3G.
If “CAZAC sequence” is used in the cellular environment, its cross-correlation is also of importance. For the purpose of suppressing interferential waves from other cells, it is desirable to assign a set of sequences of small cross-correlation values to a pilot signal between adjacent cells. The cross-correlation of “CAZAC sequence” greatly depends on its signal sequence. In other words, if the sequence length is represented by a prime number or a number including a large prime number, then the cross-correlation is very good (the cross-correlation value is small). Conversely, if the sequence length is represented by a composite number (e.g., a power of 2 or 3) made up of a small prime number only, the cross-correlation is greatly degraded (the cross-correlation value includes a large value).
Specifically, if the sequence length of “CAZAC sequence” is represented by a prime number, the cross-correlation value of arbitrary sequences is kept as 1/√N (N represents the sequence length and is currently represented by a prime number) at all times. Therefore, if the sequence length is N=127, for example, then the cross-correlation value is 1/√127 at all time, and if the sequence length is N=128, the cross-correlation value has a worse value (maximum value) of 1/√2. The number of symbols Ntx_p of a pilot signal should desirably be represented by a prime number or a number including a large prime number, but not a composite number made up of a small prime number only, such as 2, 3, 5, or the like.
If a pilot signal has a signal sequence having the characteristics of the Zadoff-Chu sequence and a prime number or a number including a large prime number is selected to represent the number of symbols thereof in order to reduce the cross-correlation of pilot signals, then the complexity of the DFT computation carried out by a transmitter for the pilot signal and the complexity of the IDFT computation carried out by a receiver for the pilot signal are very large. The complexity of the process performed by at least one of the transmitter and the receiver is thus increased. This is because, as described above, the complexities of DFT and IDFT computations are generally very large if the number of points is represented by a prime number or a number including a large prime number.